Due to their high storage density, long data retention life, and relatively low cost, optical disks have become the predominant media format for distributing information. The compact disk (CD) format was developed and marketed for the distribution of musical recordings and has replaced vinyl records. High-capacity, read-only data storage media, such as CD-ROM and DVD-ROM, have become prevalent in the personal computer field, and the DVD format may soon replace videotape as the distribution medium of choice for video information.
Relatively inexpensive optical disk writers and writable optical media are now available, making optical disks popular as backup and archival storage devices for personal computers. The large storage capacity of writable optical disks also makes them ideal for use in multimedia authoring and in other applications which require access to large amounts of storage. Current writable optical disk technologies include several write-once technologies, such as CD-Recordable (CD-R) and DVD-Recordable (DVD-R). A few technologies permit writing, erasing, and rewriting data on a disk, such as Mini-Disk (MD), CD-RW, DVD-RW, and DVD-RAM, which use magneto-optical phase-change or dye-polymer technologies. Recent advances in writable optical disk technology have made rewritable optical media more practical, and the specification for DVD-RAM calls for use of high-capacity rewritable optical media.
An optical disk is made of a transparent disk or substrate in which data, in the form of a serial bit-stream, are encoded as a series of pits in a reflective surface within the disk. The pits are arranged along a spiral or circular track. Data are read from the optical disk by focusing a low power laser beam onto a track on the disk and detecting the light reflected from the surface of the disk. By rotating the optical disk, the light reflected from the surface of the disk is modulated by the pattern of the pits rotating into and out of the field of laser illumination. Optical and imaging systems detect the modulated, reflected, laser light and produce an electrical signal that is decoded to recover the digital data stored on the optical disk. The recovered digital data, which may include error correcting codes and additional subcoded information, is further processed to recover the stored data.
To be able to retrieve data from anywhere on an optical disk, an optical system includes a pickup assembly which may be positioned to read data from any disk track. An example of an integrated optical disk assembly, wherein the laser source of illumination, focusing optics, and the detector that receives reflected illumination from the optical disk are contained within a single compact pickup assembly is described in U.S. Pat. No. 5,285,062. Processor-driven servo mechanisms are provided for focusing the optical system and for keeping the pickup assembly positioned over the track, despite disk warpage or eccentricity.
In most previously known systems the data is retrieved from the disk serially, i.e., one bit a time, so that the maximum data transfer rate for the optical disk reader is determined by the rate at which the pits pass by the pickup assembly. The linear density of the bits and the track pitch is fixed by the specification of the particular optical disk format. For example, CD disks employ a track pitch of 1.6 μm, while the DVD employs a track pitch only about one-half as wide.
Previously known methods of increasing the data transfer rate of optical disk readers have focused on increasing the rate at which the pits pass by the optical pickup assembly by increasing the rotational speed of the disk itself. Currently, drives with rotational speeds of up to 16×standard speed are commercially available, and even faster reading speeds have been achieved by moving to constant angular velocity designs. Higher disk rotational speeds, however, place increased demands on the optical and mechanical subsystems within the optical disk drive, create greater vibration, and may make such drivers more difficult and expensive to design and manufacture.
A cost effective alternative to increasing the disk rotational speed to provide faster optical disk readers is to read multiple data tracks simultaneously. Numerous methods for generating multiple beams to read several tracks simultaneously have been used. U.S. Pat. No. 5,144,616 to Yasukawa et al., for example, shows an array of laser diodes which may generate multiple beams for use in simultaneously reading multiple tracks of an optical disk. U.S. Pat. No. 4,459,690, to Corsover, uses acousto-optical techniques to split a beam into multiple beams for use in reading an optical disk. Other systems have used a diffraction grating to generate multiple beams used to simultaneously illuminate multiple tracks. The system described in commonly assigned U.S. Pat. No. 5,426,623 to Alon et al., uses a wide area illumination beam to simultaneously read multiple tracks of an optical disk.
Using a system which reads multiple tracks simultaneously may provide for dramatic increases in the speed of optical disk readers. For example, a drive which rotates the disk at only eight times the standard speed (i.e. an 8X drive), and reads seven tracks simultaneously, may provide reading speeds equivalently to a true 56X drive.
It should be noted that as used herein, a data track is a portion of the spiral data track of a typical optical disk, and follows the spiral for one rotation of the disk. Thus, a drive capable of reading multiple data tracks simultaneously will read multiple portions of the spiral data track at once. For optical disks having concentric circular tracks, a data track refers to one such circular track.
Commonly assigned U.S. Pat. No. 5,793,549 to Alon et al., describes an optical disk reader that reads multiple data tracks simultaneously, for example, using multiple laser beams. The multiple laser beams, which may be obtained by splitting a single beam using a diffraction grating or by providing multiple laser sources, are focused on and aligned with corresponding tracks of the optical disk. The reflected beams are then detected and decoded. Thus, a disk rotated at 6X the standard speed in a disk drive reading ten tracks at a time provides a data rate equivalent to a 60X single beam drive, but without the complications associated with high rotational speeds.
Implementation of simultaneous multiple track reading capability for optical disks presents new design challenges. If multiple beams are used, for example, the beams should be properly aligned with the tracks being read, and the beams reflected from the optical disk should be correctly aligned with the photodetector. For a multi-beam system, the photodetector is typically a multi-element detector such that each of the reading beams is aligned to focus on a track and projected onto one of the detector elements of the multi-element detector, as described in commonly assigned, copending U.S. patent application Ser. No. 09/464,359, filed Dec. 15, 1999.
In addition, manufacturing tolerances may lead to minor differences in the magnification of an optical pickup assembly, leading to minor differences in the spacing of the beams. There is also some variation in the track pitch allowed in the specification of most optical disk formats, such as the CD and the DVD formats. A multi-beam optical disk reader using such a format should be able to detect and correct for these magnification errors and track pitch variations to insure that the beams are correctly aligned with the tracks. When a beam is not aligned with its corresponding track, the distance between the center of the beam and the track it's supposed to focus on is referred to as a track offset. A track offset may occur due to variations in track pitch, grating angle adjustment, disk eccentricity, and sled position. In some instances, track offset can reach up half of the track pitch.
Such errors arising from magnification errors, track pitch variations, and large track offsets may result in cross-talk, which occurs when a portion of the light beam reflected from one track is being read by a photodetector associated with an adjacent track, and jitter, which occurs when the sampling of the digital signal read from a track is not done on time. The presence of cross-talk and jitter result in poor signals being read from the disk.
A previously known method for correcting for cross-talk resulting from magnification errors is described in commonly assigned U.S. Pat. No. 5,959,953 to Alon. The method involves special circuitry used in connection with a multi-beam optical pickup assembly to detect a magnification error, and to correct or compensate for the detected magnification error. One method of compensating for the detected magnification error is to perform cross-talk correction. The cross-talk correction uses additional circuitry designed to correct magnification errors or cross-talk. At present, there are no systems in place that can reduce both cross-talk and jitter without requiring additional circuitry connected to the multi-beam optical pickup assembly. In addition, the current systems are not able to reduce both cross-talk and jitter that occur due to large track offsets. The present invention is an improvement of the apparatus described above.